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CSMA/CD with the Binary Exponential Backoff
Carrier Sense Multiple Access with Collision Detection (CSMA/CD) is a network protocol for carrier transmission that operates in the Medium Access Control (MAC) layer. It senses or listens whether the shared channel for transmission is busy or not, and defers transmissions until the channel is free.
When more than one station sends their frames simultaneously, collision occurs. Binary Exponential Backoff is a collision resolution mechanism which is commonly used to schedule retransmissions after collisions in Ethernet. The waiting time that a station waits before attempting retransmission of the frame is called backoff time.
How CSMA/CD Works
CSMA/CD Algorithm
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Channel Sensing − When a frame is ready, the transmitting station checks whether the channel is idle or busy.
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Wait if Busy − If the channel is busy, the station waits until the channel becomes idle.
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Transmit and Monitor − If the channel is idle, the station starts transmitting and continually monitors the channel to detect collision.
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Collision Detection − If a collision is detected, the station starts the binary exponential backoff algorithm.
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Completion − The station resets the retransmission counters and completes frame transmission.
Binary Exponential Backoff Algorithm
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Jam Signal − The station continues transmission of the current frame for a specified time along with a jam signal, to ensure that all other stations detect collision.
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Increment Counter − The station increments the retransmission counter,
c, that denotes the number of collisions. -
Random Selection − The station selects a random number of slot times in the range 0 and 2c − 1.
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Calculate Backoff Time − If the station selects a number
kin the range 0 and 2c − 1, then:Backoff_time = k × Time_slot
where a time slot is equal to round trip time (RTT).
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Retry Transmission − At the end of the backoff time, the station attempts retransmission by continuing with the CSMA/CD algorithm.
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Abort if Maximum Reached − If the maximum number of retransmission attempts is reached, the station aborts transmission.
Backoff Examples
| Collision Number (c) | Backoff Range | Possible Wait Times |
|---|---|---|
| 1 | 0 to 21 − 1 = 0 to 1 | 0 or 1 slot times |
| 2 | 0 to 22 − 1 = 0 to 3 | 0, 1, 2, or 3 slot times |
| 3 | 0 to 23 − 1 = 0 to 7 | 0, 1, 2, ..., 7 slot times |
Key Features
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Exponential Growth − The backoff window doubles with each collision, reducing the probability of repeated collisions.
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Random Selection − Stations choose random wait times within the backoff window to avoid synchronized retransmissions.
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Fairness − All stations have equal probability of accessing the channel after the backoff period.
Conclusion
CSMA/CD with Binary Exponential Backoff effectively manages collisions in Ethernet networks by exponentially increasing the backoff window with each collision. This mechanism ensures fair channel access while reducing the probability of repeated collisions through randomized retry intervals.
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